It is well-known that, of ultra-fine particles of semiconductors or metals, nanosized particles having a smaller particle diameter than an electron wavelength (approximately 10 nm) have a profound effect of finiteness of size on electron motion, as a quantum size effect and exhibit specific physical properties differing from a bulk body (Non-patent document 1).
Core/shell-structured semiconductor nanoparticles, which are covered with a material differing from the particulate core portion, can be functionalized without altering the size or form of core particles or are expected to exhibit a characteristic differing from any of bulk materials of the core and shell so that such nanoparticles have been noted as a novel high-active catalyst, a photofunctional material or an optical element material. For instance, when the surfaces of light-emissive nanoparticles are exposed, a large number of defects existing on the nanoparticle surface becomes an emission killer, leading to reduced emission efficiency. As a result, an emission intensity can be enhanced by a coverage of a shelling material having a band gap greater than that of nanoparticles, corresponding to the emission wavelength thereof (as described in, for example, Patent documents 1-4).
Patent document 1 discloses ultra-fine particles having an insulating layer mainly composed of silicon oxide on the surface of a silicon nucleus. Patent document 2 discloses phosphor particles coated with a glass component. Patent document 3 discloses core/shell-structured particles comprised of a phosphor core of a 10 nm or less diameter and containing a first additive element to form an acceptor level and a second additive component to form a donor level in a semiconductor containing ZnS as a primary main component and partially containing or not containing a II-VI Group compound semiconductor as a secondary component, and a shell having a band gap greater than that corresponding to an emission wavelength of the phosphor. Patent document 4 discloses 2-50 nm core/shell particles comprised of a nano-sized crystalline semiconductor core and an electrically conductive shell with attached surface-modifying molecules on the particle surface.
However, a sufficiently high luminescence intensity has not been attained as yet and a blink phenomenon, so-called blinking was observed in emission of nanoparticles, for example, in single-molecule fluorescence, becoming a problem in practice.    Patent document 1: Japanese Patent Application Publication JP 5-224261A,    Patent document 2: JP 2000-265166,    Patent document 3: JP 2005-120117    Patent document 4: JP 2003-64278    Non-patent document: “Nikkei Sentangijutsu” No. 2003-1-27 pages 1-4